One method which has been used to measure the contour of aspheric optic devices involves the production of fringes by the interference of two components of a coherent light beam, one directed at the optic device to be tested and the other at a reference optic device. One system of this type includes a Twyman-Green interferometer which includes a laser light source, and a reference optic reflector device which matches the reflector optic device to be tested. A beamsplitter directs components of the laser light onto the two devices and then combines the returned components to form a fringe pattern. If the contours of the two reflective devices are close, then a pattern of fringes is produced wherein the fringes are widely spaced and substantially straight, or circular (so-called null fringes) and if there is a moderately small mismatch, then the fringes will be closer together and moderately distorted. If the workpiece device, or device to be tested, has a significant departure from a spherical surface, while the reference device is spherical, then the fringes may be too close and too distorted to be interpreted with any accuracy. If the maximum density of fringe lines is reduced, or the density of a more uniform radius of curvature (i.e., the lines do not vary from almost straight to closely curved), the fringe pattern will be easier to analyze. An approach often used to overcome the limitation is to insert an aspheric null lens (or a series of null lenses) over the test surface to reduce fringe density. This is costly and introduces the errors of the null lenses into the measurement.